The efferent motoneuron system that projects to the ear develops in mice and chicken out of the facial branchial motor primordium. Like other motor systems, the efferent cells will likely depend for their survival on unknown, neurotrophic factor(s) available at their target. In addition, various conditions such as the time of arrival and the size of the target area innervated have been suggested to play a role in the survival of motoneurons. This project seeks to test this notion by quantifying the numbers of efferents projecting to the ear over time. We will use certain features unique to the efferent system to explore issues of timing of arrival and target size which is different in the various subpopulations of efferents, and which cannot be investigated in any other motor system. We will also explore mutants with variable reductions of both the afferent and efferent innervation to certain sensory epithelia or reduction of the ear, including surgical removal, to analyze further the quantitative relationship between the developing ear and the developing efferents. Our experiments will also provide insights into the possibly limited availability of trophic support only at the otocyst by testing the survival of efferent cells rerouted to a non-otic target in chickens. Together this information will provide baseline data that can help to find clues for interventive measures in patients with loss of hair cells to prevent loss of efferents.